British Royal Navy needed more submarine mother ships according to the magazine Mitteilungen aus dem Gebiete des Seewesens dated 1906 no. 8

An item reported that the completion of the British submarines of the B-design, the increase of the displacements of the submarines and the increased number under the approved budget for this year caused a need of more mother ships. Until then were the Forth and Thames available for the submarine flotillas at respectively Devonport and Portsmouth, the cruiser Bonaventure was being converted at the Haulbowline shipyard and was then to replace the Forth at Devonport. The Forth was to be added to one of the new planned submarine stations.